In recent years, it has become common to transmit large-volume data, such as still image data and moving image data in addition to audio data in cellular mobile communication systems, in response to spread of multimedia information. Active studies associated with techniques for achieving a high transmission rate in a high-frequency radio band have been conducted to achieve large-volume data transmission.
When a high frequency radio band is utilized, however, attenuation increases as the transmission distance increases, although a higher transmission rate can be expected within a short range. Accordingly, the coverage area of a base station (which may also be referred to as “eNB”) decreases when a mobile communication system using a high frequency radio band is actually put into operation. Thus, more base stations need to be installed in this case. The installation of base stations involves reasonable costs, however. For this reason, there has been a high demand for a technique that provides a communication service using a high-frequency radio band, while limiting an increase in the number of base stations.
In order to meet such a demand, studies have been carried out on a relay technique in which a relay station (or which may also be called “relay node (RN)”) is installed between a base station and a terminal (which may also be called “user equipment (UE)”) to perform communication between the base station and mobile station via the relay station for the purpose of increasing the coverage area of each base station. The use of relay technique allows a terminal not capable of directly communicating with a base station to communicate with the base station via a relay station.
An LTE-A (long-term evolution advanced, corresponding to 3GPP Release 10) system for which the introduction of the relay technique described above has been studied is required to maintain compatibility with LIE (long term evolution, corresponding to 3GPP Release 8) in terms of a smooth transition from and coexistence with LTE. For this reason, mutual compatibility with LTE is required for the relay technique as well.
Furthermore, in an LTE-A system using a relay station (e.g., see Non-Patent Literature (hereinafter, referred to as “NPL” 1)), the relay station is required to also cover an LTE terminal. Studies are being conducted on the LTE-A system that carries out communication between a base station and a relay station (backhaul communication) and communication between the relay station and a terminal (access link) using the same frequency band. In this case, for a downlink (DL) frequency, a downlink backhaul subframe (DL BHSF) is configured as a subframe used for communication between the base station and the relay station (backhaul communication). The relay station receives a signal from the base station in a DL BHSF on the downlink and transmits a signal directed to a terminal served by the relay station (a terminal within the cell of the relay station) in subframes other than the DL BHSF. On the other hand, on the uplink (UL), an uplink backhaul subframe (UL BHSF) is configured at the fourth subframe from the DL BHSF as a subframe used for communication between the base station and the relay station (backhaul communication). On the uplink, the relay station transmits a signal directed to the base station in the UL BHSF and receives a signal from a terminal served by the relay station in a subframe other than the UL BHSF. Thus, backhaul communication (communication between the base station and the relay station) and access link communication of the relay station (communication between the relay station and the terminal) are divided in the time domain (e.g., see NPL 1).
Furthermore, in LTE, studies are being carried out on configuring a terminal served by the relay station on the downlink with an MBMS single frequency network (MBSFN) subframe. The MBSFN subframe is a subframe defined to transmit data of a multimedia broadcast multicast service (MBMS service). The terminals are configured not to receive any signal unless an MBMS service is indicated in the MBSFN subframe. Furthermore, as described above, in a BHSF (DL BHSF and UL BHSF) in which the relay station communicates with the base station, the relay station does not perform communication with terminals served by the relay station. Thus, for the LTE system, a technique is proposed that configures the MBSFN subframe with an access link subframe that overlaps with the BHSF (DL BHSF and UL BHSF) in which the relay station communicates with the base station. Such a configuration can avoid deterioration of quality measurement accuracy caused by terminals erroneously detecting signals not actually transmitted (including a common reference signal (CRS: common pilot signal)).
FIG. 1 illustrates a subframe configuration example in backhaul communication between a base station (eNB) and a relay station (RN) (communication in an eNB cell), and communication between the relay station (RN) and a terminal (UE) (communication in an RN cell).
For example, attention is focused on a leading frame shown in FIG. 1. In the downlink of the eNB cell shown in FIG. 1, subframes 1 and 3 are configured as DL BHSFs. Furthermore, on the uplink in the eNB cell shown in FIG. 1, subframes 5 and 7, the fourth subframes respectively from the subframes 1 and 3 in which the DL BHSFs are configured, configured as UL BHSFs. On the other hand, on the downlink of the RN cell shown in FIG. 1, subframes 1 and 3 configured as DL BHSFs in the eNB cell and subframes 5 and 7 configured as UL BHSFs on the uplink are configured as MBSFN subframes, respectively. The same applies to other frames shown in FIG. 1. Note that in FIG. 1, for example, DL BHSFs are configured in subframes other than subframes that cannot be configured as BHSFs (e.g., subframes to which broadcast information or the like is assigned) among subframes provided at 8-subframe intervals.
Furthermore, in the LTE-A system (e.g., see NPLs 2 to 5), a band for the LTE-A system is divided into “component carriers (component bands)” corresponding to support bandwidths of the LTE system in order to simultaneously achieve communication at an ultra-high transmission rate, as high as several times transmission rates in the LTE system, and compatibility with the LTE system. For example, the “component carrier” is a band having a maximum width of 20 MHz and is defined as a base unit (fundamental frequency band) of a communication band. Furthermore, the “component carrier” may also be denoted as “cell.” Furthermore, the “component carrier” may also be abbreviated as “CC(s).” The LTE-A system supports so-called carrier aggregation which is communication using a band with some “component carriers” thereof bundled together. In carrier aggregation, a data signal is transmitted in each CC to thereby improve the data transmission rate.
The above-described “component carrier” configured for one terminal includes one primary component carrier (or primary cell: PCell), one or a plurality of secondary component carriers (or secondary cell: SCell). For example, in a subframe in which there is no data signal to transmit by an uplink, control information such as an ACK/NACK signal for downlink data (response signal, hereinafter described as “A/N signal”) and channel quality information (channel quality indicator: CQI) are transmitted only from a PCell. More specifically, the above-described control information is transmitted using an uplink control channel (e.g., PUCCH (physical uplink control channel)) in the PCell. This is because when signals are simultaneously transmitted using different CCs on the uplink, the coverage decreases as PAPR (peak to average power ratio) increases. When downlink data is received with both the PCell and SCell in a certain subframe, the terminal transmits an A/N signal for the downlink data received in each CC in the fourth subframe from the certain subframe, using the PCell. That is, the LTE-A system (3GPP Release 10) transmits an A/N signal in the fourth subframe from the subframe in which PDSCH (physical downlink shared channel) is assigned, only from the PCell.
Furthermore, in the LTE-A system to which the aforementioned carrier aggregation is applied, the terminal may receive a plurality of downlink data items on a plurality of CCs at a time. In the LTE-A system, channel selection (also referred to as to “multiplexing”), bundling, and block coding using PUCCH format 3 are under study as methods for transmitting a plurality of A/N signals for the plurality of downlink data items.
Channel selection changes not only symbol points used for A/N signals but also resources to which the A/N signals are mapped in accordance with a pattern of error detection results relating to the plurality of downlink data items. Bundling bundles ACKs or NACKs generated from error detection results relating to the plurality of downlink data items (that is, logical A/ND of the error detection results relating to the plurality of downlink data items is calculated assuming ACK-1 and NACK=0), and transmits an A/N signal (which may also be referred to as “bundled A/N signal”) using one predetermined resource. Furthermore, according to the method for performing block coding using PUCCH format 3, the terminal collectively encodes a plurality of response signals for the plurality of respective downlink data items in blocks and transmits the coded data using a channel called “PUCCH format 3.”
For example, in the LTE-A system, when the number of A/N bits is four or less, A/N signals are transmitted from PUCCH of a PCell using channel selection, and when the number of A/N bits is five or more, A/N signals are transmitted on PUCCH of a PCell using PUCCH format 3.
In the LTE-A system, in a subframe in which there is a data signal to transmit in uplink, the above-described control information is time-multiplexed with the data signal through an uplink data channel (e.g., PUSCH (physical uplink shared channel)) and transmitted. That is, when PUSCH exists in a PCell, control information is transmitted through PUSCH of the PCell and when PUSCH exists in an SCell, control information is transmitted through PUSCH of the SCell.
In 3GPP Release 11 that further expands the LTE-A system, application of carrier aggregation to communication between a base station (eNB) and a relay station (RN) (backhaul communication) is also under study (e.g., see NPL 6).